Summary

Drops Platform.DISCORD and Platform.MATTERMOST from the hardcoded _UPDATE_ALLOWED_PLATFORMS frozenset in gateway/run.py. Both platforms are bundled plugins now (PR #24356 migrated Discord, PR #31748 migrated Mattermost) and they declare allow_update_command=True on their PlatformEntry. The gate at _handle_update_command already falls back to the registry when a platform isn't in the frozenset:

_allowed = self._UPDATE_ALLOWED_PLATFORMS
if platform not in _allowed:
    entry = platform_registry.get(platform.value)
    if not entry or not entry.allow_update_command:
        return t("gateway.update.platform_not_messaging")

Today both entries say "allowed" twice — once in the frozenset, once on the registry. Cleaning up the frozenset makes the registry flag actually do work for these platforms instead of being a redundant no-op.

What's NOT touched

• The Home Assistant entry stays — PR refactor(gateway): migrate Home Assistant adapter to bundled plugin #32500 migrates HA but isn't merged yet. Drop it in a follow-up after that lands.
• Other plugin-only platforms (teams, irc, line, google_chat, ntfy, simplex) were never in the frozenset to begin with — they've always relied on the registry fallback.
• Remaining built-ins (telegram, slack, whatsapp, signal, matrix, email, sms, dingtalk, feishu, wecom, weixin, bluebubbles, qqbot, local) stay in the frozenset; they're not plugin-migrated yet.

Verification

# All currently-migrated plugin platforms declare allow_update_command=True:
from hermes_cli.plugins import PluginManager
PluginManager().discover_and_load(force=True)
from gateway.platform_registry import platform_registry
for n in ['discord', 'mattermost', 'teams', 'irc', 'line', 'google_chat', 'ntfy', 'simplex']:
    e = platform_registry.get(n)
    print(n, e.allow_update_command)
# discord True / mattermost True / teams True / irc True / line True / google_chat True / ntfy True / simplex True

Empirical test from the worktree:

# /update on Discord/Mattermost still works (passes the gate via registry fallback)
# /update on Webhook/API_Server still blocked (not messaging platforms)

Tests

Added TestUpdateCommandPlatformGate to tests/gateway/test_update_command.py with five cases that pin down every branch of the allowlist gate. The gate previously had zero direct coverage:

• test_blocks_programmatic_interface — Platform.WEBHOOK must be blocked
• test_blocks_api_server_platform — Platform.API_SERVER must be blocked
• test_allows_plugin_platform_via_registry_fallback — Platform.DISCORD must pass via registry fallback (the regression test for this PR's change)
• test_allows_mattermost_via_registry_fallback — same for Platform.MATTERMOST
• test_allows_builtin_platform_in_allowlist — Platform.TELEGRAM must still pass directly via the frozenset

Each plugin-platform test asserts the precondition Platform.X not in _UPDATE_ALLOWED_PLATFORMS explicitly, so if anyone re-adds Discord/Mattermost to the frozenset in the future, the tests fail loudly.

All 74 /update-related tests pass (was 69 before, +5 new).

Surfaced during

Self-review of PR #32500 (Home Assistant migration). The migration playbook calls out this redundancy as out-of-scope tech debt for the migration PR itself; this PR closes that follow-up for the platforms already migrated.